Typically speaking, adhesive tapes, sticking plasters and the like are manufactured by coating films or paper with polymer solutions or suspensions. Subsequently, the organic solvents or the water are removed by drying. Especially suitable are solutions of polyacrylates in organic solvents. These polyacrylate solutions are normally manufactured by polymerizing acrylic acid, its ester and, in some cases, vinyl acetate, while adding free-radical initiators, e.g. azobisisobutyronitrile or others. After the completion of polymerization or copolymerization, organic solvents are added to dilute to a solids content of approximately 40%, after which the solution is ready for coating.
In the recent past it has been recognized that this process has a limitation, especially in cases of copolymerization, which is that the least reactive monomer escapes polymerization. For this reason, in order to arrive at a defined polymer on a reproducible basis, an excess of this least reactive monomer, e.g. vinyl acetate, is used. In principle, vinyl acetate is slower to react than the acrylates as the double bond it contains is not conjugated with a carbonyl group. The disadvantage of adding an excess of vinyl acetate is that the proportion of free vinyl acetate in the solution can amount to up to 7-8% in relation to the solids content. Because of the dilution already referred to above, this residual monomer content can be reduced to approximately 3%.
In general, residual monomer contents of this order of magnitude are undesirable in adhesives, particularly for applications in the medical and cosmetics fields and for packaging in the foods sector. In addition, maximum residual monomer concentrations are prescribed by law in many countries. Therefore, there has been an industry-wide campaign aimed a reducing the content of residual polymers in polymer solutions for the medical and foods sector.
One attempted method is known as stripping distillation. In this, the solvents which originate from the synthesis of polymers are removed almost completely. Since the physical properties of vinyl acetate and ethyl acetate (boiling point, vapor pressure) differ only little, it is necessary either to evaporate to dryness or to constantly supplement the distilled ethyl acetate. Evaporating to dryness may lead to such a high temperature load that the polymer may change. Supplementing the ethyl acetate that has been distilled off leads to a high solvent consumption and to large quantities of solvent waste, an extremely expensive process.
Another method for removing residual monomers is to react the unconverted residual monomers with extremely reactive radical initiators based on organic peroxides, which are known as scavengers. The drawback of this process is that the residual monomers are not removed but react to form oligomers that remain in the polymer.
A desirable process, therefore, should have the following advantages: it should produce a polymer with an extremely low proportion of non-converted monomers or oligomers; the thermal load during polymerization or during the removal of the solvent should be minimized; it should use only small quantities of organic solvents; it should avoid the use of scavengers; and it should be economical.